Most polymeric materials are composed of mixtures of molecules of various sizes. The distribution of molecular weights is caused by the statistical nature of the polymerization process. The mechanical behavior and processing behavior of a polymer depends to some degree on the size distribution of the macromolecules comprising the sample. The polydispersity index X.sub.w,X.sub.n, where X.sub.w and X.sub.n are the weight average and number average degrees of polymerization, respectively, is a measure of molecular weight distribution. Where the ratio is unity, all molecules are of the same molecular weight, and the polymer composition is said to be monodispersed.
The synthesis of polydimethylsiloxane-.alpha.,.omega.-diols is typically carried out by the hydrolysis and condensation of dimethyldichlorosilanes, or by the ring-opening polymerization of octamethylcyclotetrasiloxane (Lucas et al., J. Amer. Chem. Soc., 74, 5225 (1952); Patnode et al., J. Amer. Chem. Soc., 68, 358 (1946); Noll, Chemistry and Technology of Silicones, p. 192-198, Academic Press, New York, N.Y., 1968). In these cases, a mixture of cyclic and linear siloxanes is formed. In general, a normal distribution of molecular weights results. The ring-opening polymerization of hexamethylcyclotrisiloxane can give polydimethylsiloxanes with Poisson distributions. See, e.g., Kazama et al., Macromolecules 24, 122 (1991). However, the result is a mixture of dimethylsiloxane homologues HO--Si(CH.sub.3).sub.2 O--.sub.x H wherein x is a multiple of three exclusively, i.e., 3, 6, 9, etc. A polycondensation approach to narrow molecular weight distribution product has not heretofore been demonstrated.